CD (compact discs), MD (mini discs) and DVD (digital versatile discs) have been generally used as optical disks. The respective disks include a play-only type and a recordable type, and the recordable type includes a write-once type and a rewritable type.
The CDs store uncompressed sound, data and compressed image and sound data. The MDs and DVDs store compressed image and sound data mainly.
Optical disks such as CDs and DVDs are often used mainly as play-only media. Generally, a predetermined string of data is reproduced as it is. Write-once CD-Rs, which are of the recordable type, are not rewritable and need special operation for editing recorded data. Accordingly the write-once CD-Rs are used mostly for production of CDs in a small quantity or for replication of CDs. CD-RWs is of a type capable of being recorded repeatedly. However, since the number of repeated recordings is limited, it is difficult to edit recorded data freely.
Also, the CDs and DVDs have a diameter of 120 mm or 80 mm. Devices capable of recording image and/or sound data using the CDs and DVDs as recording media are larger in size and poorer in portability than devices using 4-mm wide magnetic tapes as recording media, and therefore, are significantly inconvenient to users.
On the other hand, the MDs are small in size (64 mm in diameter) and have the characteristic that the number of repeated recordings is not limited practically. However, the MDs are so formatted that a linear speed which is a relative speed of a recording head to a medium is constant and that the rotational frequency of the disks is varied depending upon a radial position. For this reason, the MDs are not suitable for high-speed access. Also, the MDs have a decreased disk diameter with keeping the same recording density as the CDs. Therefore, storage capacity is small and image data cannot be stored continuously in a large amount.
The optical disks have the characteristics that access is faster as compared with storage devices of a wind-up type such as tapes and therefore that recorded data can be rearranged at high speed according to a user's request instead of being reproduced in order.
However, the dimensions of conventional optical disks are large. There is a problem in that the use of the disks of the conventional type for recording devices results in an increased size and poorer portability of the recording devices and in that their use range is limited.
There is also a problem that, if the diameter of disks is decreased for first preference to the portability, then the storage capacity becomes insufficient and enough data cannot be recorded.
In the case where the number of repeated recordings is limited, a free editing function is restricted and it is difficult to improve the added value of data.